Depends on the system/era.

486 — usually 1.6GB. These disks are the largest in my collection that still have nice mechanical sound. And for pure DOS gaming that's more than enough. Still I plan to move to 2x1.6GB just because having system and games on separate disks should give you performance benefits.
286 — 420MB set up as 127MB. The BIOS on that system doesn't allow setting custom HDD geometry, and 127MB is the largest preset available. I also have a 42MB MFM drive from that machine but decided I prefer IDE just to avoid having to deal with disk parking and stuff like that.

Pentium Pro — 40GB drive set to 32GB via jumpers. It used to be an original Windows 95 build, so I remember setting the disk that way to avoid issues with larger disks. I think I've eventually moved to Win95 OSR2.5 but I still have that disk there.

Pentium III — 80GB. That's the biggest IDE drive I have. Can't say I'm running out of space, but I might eventually add an Ultra66 IDE controller and move to larger disks.

Depends on the games you want to play and if you want to deal with real CDs or rather play anything from hard disk. In the latter case (which I prefer), you can't have enough disk space. However, keep the BIOS limits in mind: 504 MB for CHS (up to the early 90s), 8 GB (E-CHS, no LBA), 128 GB (no 48-bit LBA). Also there's a 32 GB limit in some Windows versions, which is why many hard disks have a jumper to limit capacity to that.